Chalcogenide absorber layers for photovoltaic applications and methods of manufacturing the same

ABSTRACT

In one example embodiment, a method includes depositing one or more thin-film layers onto a substrate. More particularly, at least one of the thin-film layers comprises at least one electropositive material and at least one of the thin-film layers comprises at least one chalcogen material suitable for forming a chalcogenide material with the electropositive material. The method further includes annealing the one or more deposited thin-film layers at an average heating rate of or exceeding 1 degree Celsius per second. The method may also include cooling the annealed one or more thin-film layers at an average cooling rate of or exceeding 0.1 degrees Celsius per second.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/263,899 filed Nov. 24, 2009, which isincorporated by reference herein for all purposes.

TECHNICAL FIELD

The present disclosure generally relates to the manufacturing ofphotovoltaic devices, and more particularly, to the use of annealing informing chalcogenide absorbers for such devices.

BACKGROUND

Semiconducting chalcogenide films are typically used as absorber layersin photovoltaic devices, such as solar cells. A chalcogenide is achemical compound consisting of at least one chalcogen ion (group 16(VI) elements in the periodic table, e.g., sulfur (S), selenium (Se),and tellurium (Te)) and at least one more electropositive element. Asthose of skill in the art will appreciate, references to chalcogenidesare generally made in reference to sulfides, selenides, and telluridesonly. Thin film based solar cell devices may utilize these chalcogenidesemiconductor materials as the absorber layer(s) as is or, alternately,in the form of an alloy with other elements or even compounds likeoxides, nitrides and carbides, among others. chalcogenide (both singleand mixed) semiconductors have optical band gaps well within theterrestrial solar spectrum, and hence, can be used as photon absorbersin thin film based solar cells to generate electron hole pairs andconvert light energy to usable electrical energy.

Physical vapor deposition (PVD) based processes, and particularlysputter based deposition processes, have conventionally been utilizedfor high volume manufacturing of such thin film layers with highthroughput and yield. These thin film layers can be deposited by thesputtering (in the form of reactive/non-reactive or co-sputtering) ofhigh purity sputter targets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an equilibrium Cu—Se phase diagram.

FIG. 2 shows an equilibrium Cu—S phase diagram.

FIGS. 3A-3D illustrate example methods of annealing.

FIGS. 4A-4C illustrate alternate step-wise example methods of annealing.

FIG. 5 illustrates an example plot of X-ray diffraction data.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Particular embodiments of the present disclosure relate to the use ofannealing in forming chalcogenide absorbers for photovoltaic devices.

Copper indium gallium diselenide (e.g., Cu(In_(1-x)Ga_(x))Se₂, where xis less than or equal to approximately 0.7), copper indium galliumselenide sulfide (e.g., Cu(In_(1-x)Ga_(x))(Se_(1-y)S_(y))₂, where x isless than or equal to approximately 0.7 and y is less than or equal toapproximately 0.99), and copper indium gallium disulfide (e.g.,Cu(In_(1-x)Ga_(x))S₂, where x is less than or equal to approximately0.7), each of which is commonly referred to as a “CIGS” material, havebeen successfully used in the fabrication of thin film absorbers inphotovoltaic cells largely due to their relatively large absorptioncoefficients. In fact, photovoltaic cells having photovoltaicefficiencies greater or equal than approximately 20% have beenmanufactured using copper indium gallium diselenide absorber layers.Efforts to minimize the defect density in the absorber layer(s)(hereinafter referred to as “absorber layer” or “absorber”) have enabledthe manufacture of high quality CIGS thin film photovoltaic cells. Byway of example, reducing the defect density in the absorber layer may beachieved by heating the CIGS material close to its melting temperature,which facilitates grain growth and defect removal in the absorber layer.However, unfortunately, the melting temperature of CIGS materials isrelatively large (e.g., close to 1000 degrees Celsius) and, thus, thisapproach is generally not economical from a fabrication stand point.Furthermore, in order to use glass substrates the fabrication processcan generally not significantly exceed process temperatures ofapproximately 500 degrees Celsius.

It has been determined that, in order to manufacture photovoltaic cellshaving efficiencies at or exceeding 12%, Se and/or S have to be presentin the CIGS absorber. Unfortunately, controlling Se and S compositionsin CIGS materials has conventionally not been easy to achieve. Se and Shave low vapor pressures and, thus, can escape from Cu and In layersduring annealing or deposition at high process temperatures. In CuSe andCuS layers, this generally results in an increase in the Cu/Se or Cu/Sratios, respectively, as well as an increase in the melting point ofthese layers. By way of example, as shown in the equilibrium Cu—Se phasediagram of FIG. 1, the Cu₂Se material has a melting point over twicethat of Cu_(1-x)Se_(x) (where x is greater than or equal toapproximately 0.53). Similarly, as shown in the equilibrium Cu—S phasediagram of FIG. 2, the Cu_(1.8)S material also has a much higher meltingtemperature than Cu_(1-x)S_(x) (where x is greater than or equal toapproximately 0.5). Loss of Se and S in CIGS layers can result in thepresence of Se and S vacancies in the resultant absorber layers than candiminish the electrical performance of these CIGS absorbers.Additionally, the loss of Se and S can induce the formation of phaseswith different stoichiometry than that of copper indium galliumdiselenide and copper indium gallium disulfide. These induced phasesoften have detrimental effects on the electrical performance of CIGSabsorber layers.

One method of controlling Se or S compositions is to sputter or annealCu and In layers in the presence of H₂S and/or H₂Se. Both H₂S and H₂Seare toxic and flammable, and thus, must be handled with care. However,such a method does allow for precise dosing and very tight control ofthe chalcogenide constituent. Another method involves sputtering orannealing Cu and In layers in an atmosphere of Se or S vapors. However,thermal evaporation of Se and S is conventionally not easy to control inhigh throughput fabrication processes. The sulfurization/selenizationoccurs in an environment of excess chalcogenide and cannot be preciselydosed or controlled. Furthermore, to minimize Se or S loss, the Cu andIn layers can be rapidly annealed. By way of example, in a rapidannealing process, the temperature of the substrate upon which thephotovoltaic cells are deposited/grown may be increased a few degreesCelsius per second (or faster) to minimize Se or S evaporation.

In particular embodiments, a CIGS absorber layer is formed by annealingCu and/or In containing thin films. In some embodiments, the annealingincludes pulsed or flash annealing. By way of example, the annealingprocess of particular embodiments is performed on one of the fourexample multilayer structures described below.

The first example multilayer structure comprises[Cu/In_(1-x)Ga_(x)]_(N)/Se_(1-y-z)S_(y)Te_(z) (where, in particularembodiments, x≦0.7, 0≦y≦1, 0≦z≦1, N≦100) multilayers. In particularembodiments, the thickness of each layer in the multilayer structure mayrange from 0 to 4 μm while the total thickness of the N layer structureis less than approximately 8 μm. By way of example, the following layerstructures may be used for the subsequent annealing process ofparticular embodiments:

-   -   a) [Cu (0.15-0.5 μm thickness)/In_(1-x)Ga_(x) (0.3-1.1 μm        thickness)]_(N)/Se_(1-y)S_(y)(0.6-4.0 μm thickness),    -   b) Cu (0.15-0.5 μm thickness)/In_(1-x)Ga_(x) (0.3-1.1 μm        thickness)/Se_(1-y-z)S_(y)Te_(z) (0.6-4.0 μm thickness),    -   c) [In_(1-x)Ga_(x) (0.3-1.1 μm thickness)/Cu (0.15-0.5 μm        thickness)]_(N)/Se_(1-y) S_(y)(0.6-4.0 μm thickness),    -   d) In_(1-x)Ga_(x) (0.3-1.1 μm thickness)/Cu (0.15-0.5 μm        thickness)/Se_(1-y-z)S_(y)Te_(z) (0.6-4.0 μm thickness),    -   e) In_(1-x)Ga_(x) (0.15-0.55 μm thickness)/Cu (0.08-0.25 μm        thickness)/In_(1-x)Ga_(x) (0.15-0.55 μm thickness)/Cu (0.08-0.25        μm thickness)/Se_(1-y-z)S_(y)Te_(z) (0.6-4 μm thickness),    -   f) In_(1-x)Ga_(x) (0.25-1.0 μm thickness)/Cu (0.15-0.5 μm        thickness)/In_(1-x)Ga_(x) (0.03-0.11 μm        thickness)/Se_(1-y-z)S_(y)Te_(z) (0.6-4 μm thickness),    -   g) In_(1-x)Ga_(x) (0.25-1.0 μm thickness, Ga concentration        0.2≦x≦0.5)/Cu (0.15-0.5 μm thickness)/In_(1-x)Ga_(x) (0.03-0.11        μm thickness, Ga concentration 0.1≦x≦0.3)/Se_(1-y-z)S_(y)Te_(z)        (0.6-4 μm thickness).

The second example multilayer structure comprises[(In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)/Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β)]_(N)(where, in particular embodiments, x≦0.7, α≦0.8, β≦0.8, 0≦y≦1, 0≦z≦1,N≦100) multilayers. In particular embodiments, the thickness of eachlayer in the multilayer structure may range from 0 to 6 μm while thetotal thickness of the N layer structure is less than approximately 8μm. By way of example, following layer structures may be used for thesubsequent annealing process of particular embodiments:

-   -   a) [(In_(1-x)Ga_(x))_(1-α)(Se_(1-y)S_(y))_(α) (0.5-2.5 μm        thickness)/Cu_(1-β)(Se_(1.y)S_(y))_(β) (0.3-2 μm        thickness)]_(N),    -   b) (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) (0.5-2.5 μm        thickness)/Cu_(1-β)(Se_(1.y-z)S_(y)Te_(z))_(β) (0.3-2 μm        thickness),    -   c) (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) (0.45-2.25        μm thickness)/Cu_(1-β)(Se_(1.y-z)S_(y)Te_(z))_(β) (0.3-2 μm        thickness)/(In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)        (0.05-0.25 μm thickness),    -   d) (In_(1-x)Ga_(x))_(1-α)(Se_(1.y-z)S_(y)Te_(z))_(α) (0.45-2.25        μm thickness, Ga concentration        0.2≦x≦0.5)/Cu_(1-β)(Se_(1.y-z)S_(y)Te_(z))_(β) (0.3-2 μm        thickness)/(In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)        (0.05-0.25 μm thickness, Ga concentration 0.1≦x≦0.3),    -   e) [(In_(1-x)Ga_(x))_(1-α)(Se_(1-y)S_(y))_(α) (0.45-2.25 μm        thickness, Ga concentration        0.2≦x≦0.5)/Cu_(1-β)(Se_(1.y)S_(y))_(β) (0.3-2 μm        thickness)/(In_(1-x)Ga_(x))_(1-α)(Se_(1-y)S_(y))_(α) (0.05-0.25        μm thickness, Ga concentration 0.1≦x≦0.3)]_(N).

The third example multilayer structure comprisesCu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β), (where, inparticular embodiments, 0.1≦α≦0.4, 0.1≦β≦0.4, α+β≦0.7, x≦0.7, x≦0.7,0≦y≦1, 0≦z≦1) layer (for 0.5<α+β≦0.7 the layer is annealed in thepresence of H₂S and/or H₂Se). In particular embodiments, the thicknessof the layer can range from 0.8 to 8 μm. By way of example, followinglayer structures may be used for the subsequent annealing process ofparticular embodiments:

-   -   a)        Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y)S_(y))_(1-α-β)(0.18≦α≦0.25,        0.24≦β≦0.28, α+β≦0.5, x≦0.7, 0≦y≦1),    -   b) Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, x≦0.7, 0≦y≦1, z≦0.1),    -   c) Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.22≦α≦0.3, 0.17≦β≦0.25, α+β≦0.5, x≦0.7, 0≦y≦1, z≦0.1),    -   d) Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.2≦α≦0.25, 0.23≦β≦0.28, α+β≦0.5, x≦0.7, y≦0.4, z=0).

The fourth example multilayer structure comprises[Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)]_(N), (where,in particular embodiments, for each layer 0≦α≦1, 0≦β≦1, x≦0.7, 0≦y≦1,0≦z≦1, total concentration of Cu, In+Ga, and Se+S+Te across all N layersshould not exceed 30 at. %, 30 at. % and 70 at. %, respectively andtotal number of layers N≦100). In particular embodiments, the thicknessof each layer in the multilayer structure may range from 0 to 6 μm whilethe total thickness of the N layer structure is less than approximately8 μm. By way of example, following layer structures may be used for thesubsequent annealing process of particular embodiments:

-   -   a) Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y)S_(y))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, 0.2≦x≦0.5, 0≦y≦1) (0.3-2 μm        thickness)/Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1.y)S_(y))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, 0.1≦α≦0.3, 0≦y≦1) (0.2-2 μm        thickness)/Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y)S_(y))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, 0≦x≦0.25, 0≦y≦1) (0.3-2 μm        thickness),    -   b) Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, 0.2≦x≦0.5, 0≦y≦1,        z≦0.1)(0.3-2 μm        thickness)/Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β≦0.28, α+β≦0.5, 0.1≦x≦0.3, 0≦y≦1, z≦0.1)        (0.3-2 μm        thickness)/Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)        (0.18≦α≦0.25, 0.2≦β0.28, α+β≦0.5, 0≦x≦25, 0≦y≦1, z≦0.1) (0.3-2        μm thickness).

In particular embodiments, the multilayer structures described above maybe deposited (e.g., by conventional sputtering or magnetron sputtering)in vacuum or in an atmosphere that consists of or includes at least oneof the following gases: Ar, H, N₂, O₂, H₂S, and H₂Se. In particularembodiments, one or more of the layers of the multilayer structuresdescribed above may be doped (e.g., up to approximately 4 atomic percent(atomic %)) with at least one of the following elements: Na, P, K, N, B,As, and Sb. In particular embodiments, to improve the electricalproperties of the resultant CIGS absorbers and to optimize thesubsequent annealing process, Cu and Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β)layers may contain up to approximately 20 atomic % of at least one ofthe following elements: Al, Si, Ti, V, Zn, Ga, Zr, Nb, Mo, Ru, Pd, In,Sn, Ta, W, Re, Ir, Pt, Au, Pb, and Bi. In particular embodiments,In_(1-x)Ga_(x) and (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)layers may contain up to approximately 20 atomic % of at least one ofthe following elements: Al, Si, Ti, V, Cu, Zn, Zr, Nb, Mo, Ru, Pd, Sn,Ta, W, Re, Ir, Pt, Au, Pb, and Bi. In particular embodiments,Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(γ) layers may containup to approximately 20 atomic % of at least one of the followingelements: Al, Si, Ti, V, Zn, Zr, Nb, Mo, Ru, Pd, Sn, Ta, W, Re, Ir, Pt,Au, Pb, and Bi. In particular embodiments, if, in the proposed layerstructures, the total concentration of Cu is larger than the totalconcentration of In and Ga, the layers may be etched (e.g., using KCNfor etching Cu-rich phases) after the subsequent annealing process toremove Cu-rich phases, which may be detrimental for CIGS performance.

In particular embodiments, all of the layers described above, exceptSe_(1-y-z)S_(y)Te_(z), may be deposited by magnetron sputtering. Inparticular embodiments, the Se_(1-y-z)S_(y)Te_(z) may be deposited usingthermal evaporation techniques. In particular embodiments, Cu,In_(1-x)Ga_(x), (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α),Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β), andCu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(γ) layers may bedeposited over either non-heated substrates or over substrates that havebeen pre-heated to temperatures up to, by way of example, 12 degreesCelsius. By way of example, the following substrate temperatureconditions may be used during the sputtering of Cu, In_(1-x)Ga_(x),(In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α),Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β), andCu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(γ) layers:

-   -   a) The substrate temperature is kept below approximately 200        degrees Celsius during layer deposition to minimize the        evaporation of S, Se, and Te from the deposited multilayer        stack,    -   b) The substrate temperature is first kept below approximately        200 degrees Celsius for sputtering at least one of the layers        and then is subsequently increased to temperatures between, by        way of example, 200 degrees Celsius and 600 degrees Celsius for        sputtering the remaining layers (e.g.,        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) is deposited        at approximately 200 degrees Celsius and then        Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β) is deposited over        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α), at a        temperature of approximately 550 degrees Celsius,    -   c) The substrate temperature is kept between approximately 200        and 400 degrees Celsius during layer deposition,    -   d) The substrate temperature is first kept below approximately        450 degrees Celsius for sputtering at least one layer and then        is increased to temperatures between, by way of example, 400 and        700 degrees Celsius for sputtering the remaining layers,    -   e) The substrate temperature is first kept below 450 degrees        Celsius for sputtering at least one layer, then is increased to        temperatures between, by way of example, 400 and 700 degrees        Celsius for sputtering at least one layer, and then is decreased        to temperatures between, by way of example, 550 and 100 degrees        Celsius for sputtering the remaining layers (e.g.,        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) is deposited        at approximately 200 degrees Celsius, then        Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β) is deposited over        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) at a        temperature of approximately 550 degrees Celsius, and then        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α) is deposited        over        (In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)/Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β)        layers at approximately 500 degrees Celsius.

In particular embodiments, the annealing of the multilayer thin filmstructures described above can be performed using a light source, suchas, by way of example, a halogen lamp or a laser, as well asadditionally or alternately, using resistive heaters. The heating may beeffected either directly onto the surface of the multilayer thin filmstructure or by way of the back substrate. By way of example, FIGS.3A-3D illustrate various methods of annealing the multilayer structuresdescribed above. More particularly, FIGS. 3A, 3C, and 3D show simplifiedplots of the dependence of the temperature of a multilayer structure asa function of time (T(t)) during annealing of the multilayer structure.Even more particularly, in FIG. 3A, the temperature of the multilayerstructure is first increased from T₀ to T₁ with a temperature ramp rate(increase rate) of (T₁−T₀)/(t₁−t₀) followed by a decrease to T₀ with acooling rate of (T₀−T₁)/(t₂−t₁). FIG. 3B shows a more realisticrepresentation of the temperature dependence (T(t)) of the multilayerstructure during the annealing process. More specifically, as shown inFIG. 3B, the ramp rate usually decreases at higher temperatures and thecooling rate is usually faster at higher temperatures. However, for somecases, a linear temperature dependence is assumed for simplicityreasons. Continuing with the example annealing process shown in FIG. 3C,the temperature of the multilayer structure is first increased from T₀to T₁ with a temperature ramp rate of (T₁−T₀)/(t₁−t₀). The temperatureof the multilayer structure is then kept at approximately T₁ for a timet₂−t₁ before subsequently reducing the temperature to T₀ with a coolingrate of (T₀−T₁)/(t₃−t₂). Finally, in the example annealing process shownin FIG. 3D, the multilayer structure is first preheated to a temperatureT₁ before increasing the temperature of the multilayer structure from T₁to T₂ with a temperature ramp rate of (T₂−T₁)/(t₂−t₁). The temperatureof the multilayer structure is then kept at approximately T₂ for a timet₃−t₂ before subsequently reducing the temperature to T₀ with a coolingrate of (T₀−T₂)/(t₄−t₃).

FIGS. 4A-4C illustrate various alternate step-wise methods of annealingthe multilayer structures described above. More particularly, FIG. 4Aillustrates an annealing process in which the multilayer structure isfirst heated to T₁ with a ramp rate of (T₁−T₀)/(t₁−t₀), then kept atapproximately T₁ for a time t₂−t₁, then heated to T₂ with a ramp rate of(T₂−T₁)/(t₃−t₂), then kept at approximately T₂ for a time t₄−t₃, and soon until a target temperature T_(n) is reached. FIG. 4B illustrates anannealing process in which the multilayer structure is first heated tothe highest target temperature T₁ where annealing is performed at T₁ fora time t₂−t₁, followed by step-wise cooling. More particularly, thetemperature of the multilayer structure is decreased to T₂ at a rate(T₂−T₁)/(t₃−t₂) followed by maintaining the temperature at approximatelyT₂ for a time t₄−t₃ and so on until a target temperature T₀ is reached.FIG. 4C illustrates an annealing process in which the multilayerstructure is heated using the step-wise heating method described withreference to FIG. 4A and then subsequently cooled using the step-wisecooling method described with reference to FIG. 4B.

In particular embodiments, during the annealing process, the multilayerstructure is annealed according to one of the following more specificmethods. In particular embodiments, the annealing comprises pulsed orflash annealing.

-   -   1) The multilayer structure is heated with a heating ramp rate        of or exceeding 1 degree Celsius per second to a highest        temperature below approximately 1200 degrees Celsius, followed        by cooling at a cooling rate of or exceeding 0.1 degrees Celsius        per second to a temperature below, for example, 300 degrees        Celsius. However, in embodiments in which the annealing process        utilizes laser annealing, the heating ramp rate may exceed, for        example, 10⁶ degrees Celsius per second.    -   2) The multilayer structure is heated with a heating ramp rate        exceeding 1 degree Celsius per second to a highest temperature        below approximately 1200 degrees Celsius, followed by cooling at        a cooling rate exceeding 1 degree Celsius per second to a        temperature below 300 degrees Celsius.    -   3) The multilayer structure is heated with a heating ramp rate        exceeding 1 degree Celsius per second to a highest temperature        below approximately 1200 degrees Celsius, then kept at this        temperature for less than 60 minutes, followed by cooling at a        cooling rate of or exceeding 0.1 degree Celsius per second to a        temperature below 300 degrees Celsius.    -   4) The multilayer structure is heated with a heating ramp rate        exceeding 1 degree Celsius per second to a highest temperature        below approximately 650 degrees Celsius, then kept at this        temperature for less than 60 minutes, followed by cooling at a        cooling rate exceeding 0.1 degree Celsius per second to a        temperature below 300 degrees Celsius.    -   5) The multilayer structure is first pre-heated to a temperature        below approximately 400 degrees Celsius, then heated with a        heating ramp rate exceeding 1 degree Celsius per second to a        highest temperature below approximately 1200 degrees Celsius,        then kept at this temperature for less than 60 minutes, followed        by cooling at a cooling rate exceeding 0.1 degree Celsius per        second to a temperature below 300 degrees Celsius.    -   6) The multilayer structure is heated with a heating ramp rate        exceeding 1 degree Celsius per second to a highest temperature        below approximately 1200 degrees Celsius, then kept at this        temperature for less than 60 minutes, then cooled at a cooling        rate exceeding 0.1 degree Celsius per second to a temperature        below 600 degrees Celsius, then kept at this temperature for        less than 60 minutes, then cooled with a cooling rate exceeding        0.1 Celsius per second to a temperature below 300 degrees        Celsius.    -   7) The multilayer structure is first pre-heated to a temperature        below approximately 400 degrees Celsius, then heated with a        heating ramp rate exceeding 1 degree Celsius per second to a        highest temperature below approximately 1200 degrees Celsius,        then kept at this temperature for less than 60 minutes, then        cooled at a cooling rate exceeding 0.1 degree Celsius per second        to a temperature below approximately 600 degrees Celsius, then        kept at this temperature for less than 60 minutes, then cooled        with a cooling rate exceeding 0.1 Celsius per second to a        temperature below 300 degrees Celsius.    -   8) The multilayer structure is first pre-heated to a temperature        below approximately 400 degrees Celsius, then heated with a        heating ramp rate exceeding 1 degree Celsius per second to a        highest temperature below approximately 650 degrees Celsius,        then kept at this temperature for less than 60 minutes, then        cooled at a cooling rate exceeding 0.1 degree Celsius per second        to a temperature below approximately 560 degrees Celsius, then        kept at this temperature for less than 60 minutes, then cooled        with a cooling rate exceeding 0.1 Celsius per second to a        temperature below 300 degrees Celsius.

In particular embodiments, the annealing processes described above maybe performed in vacuum or in the presence of an atmosphere of gas. Byway of example, the atmosphere of gas may include or consist of at leastone of H, He, N₂, O₂, Ar, H₂S, Kr, H₂Se, or Xe. In particularembodiments, the pressure of the gas atmosphere may range from, by wayof example, 1E-8 Pa to approximately 1E7 Pa. In an alternate embodiment,the multilayer structures described above may first be annealed invacuum followed by annealing in the presence of at least one gas as justdescribed.

In one particular embodiment, a multilayer structure comprisingIn_(1-x)Cu_(x) (where x is less than or equal to approximately 0.5) andIn_(1-x)Cu_(x) (where x is greater than or equal to approximately 0.5)is sputtered in the presence of a reactive H₂S atmosphere atapproximately 500 degrees Celsius. In one experiment, the totalcomposition of Cu in the resultant multilayer structure was higher thanthat of In resulting in the formation of CuS phases in conjunction withCuInS2. By way of example, as illustrated in FIG. 5, which plots X-raydiffraction data, the resultant multilayer structure shows the formationof chalcopyrite ordered (signature peak (110) at 17.8°) and CuAu ordered(signature peak (001) at 16°) CuInS₂ phases in the film structure. Thechalcopyrite ordering of CuInSe₂ may generally be more attractive forhigh efficiency CIGS based photovoltaic cells. This multilayer structuremay then be annealed with, by way of example, a halogen lamp at aheating ramp rate of approximately 3 degrees Celsius per second to amaximum annealing temperature of approximately 550 degrees Celsius andwith an annealing time at the maximum annealing temperature ofapproximately 2 minutes. In one experiment, after such annealing of thismultilayer structure, the majority of the CuAu phase was transformedinto a chalcopyrite phase. This is evidenced by the X-ray spectra ofFIG. 5, which shows a reduction in the intensity of the CuAu-orderingpeak (peak (001) at 16°) and an increase of the intensity of thechalcopyrite peak (peak (110) at 17.8°). Additionally, the experimentdemonstrated that the majority of the CuS phase evaporated from the filmstructure. As those of skill in the art may appreciate, CuS is highlyconductive and, hence, its presence in CIGS absorbers is detrimental tothe performance of photovoltaic cells. Thus, annealing in accordance toparticular embodiments described above, can improve the ordering inCuInS₂ (as well as in CuInSe₂) and, furthermore, remove the CuS phasefrom the film, both of which improve the quality of the CIGS absorber ina photovoltaic cell.

As another example, a (In_(1-x)Ga_(x))₂Se₃/Cu₂Se (where x is less thanor equal to approximately 0.7) multilayer structure may be sputtered attemperatures below approximately 400 degrees Celsius, followed byannealing under the following conditions. First, the multilayerstructure is heated with a ramp rate exceeding 1 degree Celsius persecond to a maximum annealing temperature below approximately 600degrees Celsius. The multilayer structure may then be held atapproximately the maximum temperature for less than approximately 40minutes. The multilayer structure may then be cooled at a cooling rateexceeding approximately 0.1 degrees Celsius per second. This annealingprocess may be performed in an atmosphere that consists of at least oneof He, Ar, N₂, H₂S, and H₂Se. The gas pressure may be an importantparameter and may be varied from, by way of example, 1E-8 Pa (vacuum) to1E7 Pa. In an even more particular example, the(In_(1-x)Ga_(x))₂Se₃/Cu₂Se multilayer structure may be annealed below600 degrees Celsius in vacuum for less than 30 minutes followed bycooling in less than 14 Pa of a H₂S atmosphere at a cooling rateexceeding 0.1 degrees Celsius per second to a temperature below 300degrees Celsius.

As another example, a (In_(1-x)Ga_(x))₂Se₃/CuSe₂ (where x is less thanor equal to approximately 0.7) multilayer structure may be sputtered attemperatures below approximately 400 degrees Celsius, followed byannealing under the following conditions. First, the multilayerstructure is heated with a ramp rate exceeding 1 degree Celsius persecond to a maximum annealing temperature below approximately 600degrees Celsius. The multilayer structure may then be held atapproximately the maximum temperature for less than approximately 40minutes. The multilayer structure may then be cooled at a cooling rateexceeding approximately 0.1 degrees Celsius per second to a temperaturebelow approximately 550 degrees Celsius. The multilayer structure maythen be held at approximately this temperature for less than 40 minutesfollowed by cooling at a cooling rate exceeding 0.1 degrees Celsius persecond to a temperature below approximately 300 degrees Celsius. Thisannealing process may be performed in an atmosphere that consists of atleast one of He, Ar, N₂, H₂S, and H₂Se. Again, the gas pressure may bean important parameter and may be varied from, by way of example, 1E-8Pa (vacuum) to 1E7 Pa. In an even more particular example, the(In_(1-x)Ga_(x))₂Se₃/CuSe₂ multilayer structure may be annealed below600 degrees Celsius in vacuum for less than 30 minutes followed bycooling in vacuum to a temperature below 550 degrees Celsius, thenannealed at approximately this temperature for less than 30 minutes inless than 14 Pa of a H₂S atmosphere, followed by cooling at a coolingrate exceeding 0.1 degrees Celsius per second to a temperature below 300degrees Celsius.

As another example, a In_(1-x)Ga_(x)/Cu₂Se/CuSe₂ (where x is less thanor equal to approximately 0.7) multilayer structure may be annealedunder the following conditions. First, the multilayer structure isheated with a ramp rate exceeding 1 degree Celsius per second to amaximum annealing temperature below approximately 600 degrees Celsius.The multilayer structure may then be held at approximately the maximumtemperature for less than approximately 40 minutes. The multilayerstructure may then be cooled at a cooling rate exceeding approximately0.1 degrees Celsius per second. Again, this annealing process may beperformed in an atmosphere that consists of at least one of He, Ar, N₂,H₂S, and H₂Se, and again, the gas pressure may be an important parameterand may be varied from, by way of example, 1E-8 Pa (vacuum) to 1E7 Pa.

As another example, a Cu(In_(1-x)Ga_(x))S (where x is less than or equalto approximately 0.7) multilayer structure may be annealed under thefollowing conditions. First, the multilayer structure is heated with aramp rate exceeding 1 degree Celsius per second to a maximum annealingtemperature below approximately 600 degrees Celsius. The multilayerstructure may then be held at approximately the maximum temperature forless than approximately 40 minutes. The multilayer structure may then becooled at a cooling rate exceeding approximately 0.1 degrees Celsius persecond. Again, this annealing process may be performed in an atmospherethat consists of at least one of He, Ar, N₂, H₂S, and H₂Se, and again,the gas pressure may be an important parameter and may be varied from,by way of example, 1E-8 Pa (vacuum) to 1E7 Pa.

In particular embodiments, shorter annealing times are used to minimizethe production time for the resultant CIGS absorbers. Moreover, shorterannealing times, as described above, can minimize the loss of S, Se, andTe in these absorbers.

Additionally, a number of process enhancements may also be implementedin particular embodiments to minimize and/or compensate for chalcogenidedepletion during processing and/or annealing. By way of example, giventhe precise control that can be achieved in the previously describedprocess pathways, some embodiments may start off with an excess of thechalcogenide in the absorber layer (in either elemental or compoundforms) such that the depletion that occurs during thermal processing isprecisely accounted for and, thus, the processing results in a CIGSabsorber having the desired end stoichiometry. As another example, someembodiments may include the simultaneous thermal processing of twosamples (two multilayer CIGS structures) positioned in contact (or inclose proximity) face-to-face with one another such that the evaporativelosses of S, Se, and Te of each sample are minimized due to the materialconfinement and local over-pressure that results.

The present disclosure encompasses all changes, substitutions,variations, alterations, and modifications to the example embodimentsherein that a person having ordinary skill in the art would comprehend.Similarly, where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend.

1. A method comprising: depositing a first film layer onto a substrate,wherein the film layer includes one or more of Se, Te and S, and eitherCu or one or more of In and Ga, and wherein the one or more of Se, Teand S in the aggregate is less than or equal to 80 atomic percent;depositing a second film layer onto the first film layer, wherein thesecond film layer comprises Cu if the first film thin contains In and/orGa, or In and/or Ga if the first film layer contains Cu; depositing athird film layer over the first and second film layers, wherein thethird film layer comprises over 20 atomic percent of Se, Te and/or S inthe aggregate; annealing the first, second and third deposited filmlayers at an average heating rate of or exceeding 1 degree Celsius persecond; removing excess material from one or more of the first, second,or third film layers, the excess material comprising Cu and one or moreof S, Se, or Te; and cooling the annealed first, second and third filmlayers at an average cooling rate of or exceeding 0.1 degrees Celsiusper second.
 2. The method of claim 1 wherein at least one of the filmlayers comprises [Cu/In_(1-x)Ga_(x)]_(N)/Se_(1-y-z)S_(y)Te_(z) (wherex≦0.7, 0≦y≦1, 0≦z≦1, N≦100).
 3. The method of claim 1 wherein at leastone of the film layers comprises[(In_(1-x)Ga_(x))_(1-α)(Se_(1-y-z)S_(y)Te_(z))_(α)/Cu_(1-β)(Se_(1-y-z)S_(y)Te_(z))_(β)]_(N)(where x≦0.7, α≦0.8, β≦0.8, 0≦y≦1, 0≦z≦1, N≦100).
 4. The method of claim1 wherein at least one of the film layers comprisesCu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β), (where0.1≦α≦0.4, 0.1≦β≦0.4, α+β≦0.7, x≦0.7, 0≦y≦1, 0≦z≦1) layer.
 5. The methodof claim 1 wherein the one or more film layers are deposited in vacuum.6. The method of claim 1 wherein the one or more film layers aredeposited in an atmosphere that consists of or includes at least one ofthe following gases: Ar, H, N₂, O₂, H₂S, and H₂Se.
 7. The method ofclaim 1 wherein at least one of the one or more film layers are dopedwith at least one of the following elements: Na, P, K, N, B, As, and Sb.8. The method of claim 1 wherein annealing the one or more depositedfilm layers comprises pre-heating the one or more deposited film layersto a first temperature, wherein the one or more deposited film layers isheated to a second temperature at the average heating rate.
 9. Themethod of claim 1 wherein annealing the one or more deposited filmlayers is performed in vacuum.
 10. The method of claim 1 whereinannealing the one or more deposited film layers is performed in thepresence of an atmosphere of gas.
 11. The method of claim 1 whereinannealing the one or more deposited film layers is performed in vacuumfollowed by annealing in the presence of at least one gas.
 12. Themethod of claim 1 wherein the atomic percent of Se, Te and/or S in theaggregate increases from the first to the third deposited film layer.13. The method of claim 1 wherein the third film layer contains lessthan or equal to 25 atomic percent of In and Ga in the aggregate. 14.The method of claim 1 wherein annealing is performed using an externalheating source.
 15. The method of claim 1 wherein annealing is performedusing pulse or flash annealing.
 16. The method of claim 1 whereinannealing is performed at an average heating rate of approximately 1degree Celsius per second.
 17. The method of claim 1 wherein annealingis performed at an average heating rate of less than 10 degrees Celsiusper second.
 18. The method of claim 1 wherein cooling is performed at anaverage cooling rate of approximately 0.1 degree Celsius per second. 19.The method of claim 1 wherein: annealing comprises heating the depositedfilm layers to a first temperature below approximately 1200 degreesCelsius; and cooling comprises cooling the film layers to a secondtemperature below 300 degrees Celsius.
 20. The method of claim 1wherein: annealing comprises heating the deposited film layers to afirst temperature below approximately 1200 degrees Celsius; and coolingcomprises cooling the film layers at an average cooling rate of orexceeding 1 degree Celsius per second to a second temperature below 300degrees Celsius.
 21. The method of claim 1 wherein: annealing comprisesheating the deposited film layers to a first temperature belowapproximately 1200 degrees Celsius and holding at the first temperaturefor less than 60 minutes; and cooling comprises cooling the film layersto a second temperature below 300 degrees Celsius.
 22. The method ofclaim 1 wherein: annealing comprises heating the deposited film layersto a first temperature below approximately 650 degrees Celsius andholding at the first temperature for less than 60 minutes; and coolingcomprises cooling one or more of the film layers to a second temperaturebelow 300 degrees Celsius.
 23. The method of claim 1 wherein: annealingcomprises pre-heating the deposited film layers to a first temperaturebelow approximately 400 degrees Celsius, heating the deposited filmlayers to a second temperature below approximately 1200 degrees Celsius,and holding at the second temperature for less than 60 minutes; andcooling comprises cooling the film layers to a third temperature below300 degrees Celsius.
 24. The method of claim 1 wherein: annealingcomprises heating the deposited film layers to a first temperature belowapproximately 1200 degrees Celsius; and cooling comprises cooling thefilm layers to a second temperature below 600 degrees Celsius, holdingat the second temperature for less than 60 minutes, and then cooling thefilm layers to a third temperature below 300 degrees Celsius.
 25. Themethod of claim 1 wherein: annealing comprises pre-heating the depositedfilm layers to a first temperature below approximately 400 degreesCelsius, heating the deposited film layers to a second temperature belowapproximately 1200 degrees Celsius, and holding at the secondtemperature for less than 60 minutes; and cooling comprises cooling thefilm layers to a third temperature below approximately 600 degreesCelsius, holding at the third temperature for less than 60 minutes, andcooling to a fourth temperature below 300 degrees Celsius.
 26. Themethod of claim 1 wherein: annealing comprises pre-heating the depositedfilm layers to a first temperature below approximately 400 degreesCelsius, heating the deposited film layers to a second temperature belowapproximately 650 degrees Celsius, and holding at the second temperaturefor less than 60 minutes; and cooling comprises cooling the film layersto a third temperature below approximately 560 degrees Celsius, holdingat the third temperature for less than 60 minutes, and cooling to afourth temperature below 300 degrees Celsius.
 27. The method of claim 1wherein one or more of the first, second, or third film layers has athickness of 0.15 to 8.0 μm.
 28. The method of claim 1 wherein one ormore of the first, second, or third film layers comprises In_(1-x)Cu_(x)(where x>0.5).
 29. The method of claim 1 wherein annealing the first,second, and third deposited film layers results in the formation ofCu(In, Ga)(S, Se, Te)₂ with a tetragonal chalcopyrite crystal structure.30. The method of claim 1 wherein removing the excess material occursduring annealing.
 31. The method of claim 1 wherein the excess materialcomprises Cu_(1-x)S_(x) (where 0.2≦x≦1).
 32. The method of claim 1wherein the excess material comprises Cu_(1-x)Se_(x) (where 0.2≦x≦1).33. The method of claim 1 wherein the excess material comprisesCu_(1-x)(S, Se)_(x) (where 0.2≦x≦1).
 34. The method of claim 1 whereinthe excess material comprises Cu_(1-x)(S, Se, Te)_(x) (where 0.2≦x≦1).35. The method of claim 1 wherein the excess material comprisesCu_(1-x)(Se_(1-y)S_(y)Te_(z))_(x) (where 0.1≦x<1, 0≦y≦1, 0≦z≦1).
 36. Themethod of claim 2 wherein at least one of the film layers comprises upto approximately 20 atomic % of at least one of the following elements:Al, Si, Ti, V, Zn, Ga, Zr, Nb, Mo, Ru, Pd, In, Sn, Ta, W, Re, Ir, Pt,Au, Pb, and Bi.
 37. The method of claim 2 wherein at least one of thefilm layers comprises[Cu_(α)(In_(1-x)Ga_(x))_(β)(Se_(1-y-z)S_(y)Te_(z))_(1-α-β)]_(N), (wherefor each layer 0≦α≦1, 0≦β≦1, x≦0.7, 0≦y≦1, 0≦z≦1, and totalconcentration of Cu, In and Ga, and Se+S+Te across all N layers does notexceed 30 at. %, 30 at. % and 70 at. %, respectively and a total numberof layers N≦100).
 38. The method of claim 4 wherein, for 0.5<α+β≦0.7,the film layer is annealed in the presence of H₂S and/or H₂Se.